1. Field of the Invention
The present invention relates to a controller for an injection molding machine.
2. Description of the Related Art
An in-line-screw injection molding machine is configured to perform a metering process, injection process, and packing process. In the metering process, a screw in a heating cylinder is rotated as it is retracted, whereby a molten resin is force-fed to the distal end portion of a heating cylinder. In the injection process, the screw is advanced thereafter to fill the molten resin into a mold.
In the injection process, the screw is advanced based on preset injection stroke and velocity to fill the molten resin in the heating cylinder into the mold. When the screw advances to an injection-packing switching position, the injection process is switched to the packing process.
In the packing process, pressure control is performed based on a preset packing pressure and a set time, whereby the molten resin is fully filled into the mold, and moreover, shrinkage cavity of molded products due to contraction of the resin is compensated. If resin flow passages in the mold are narrow or in order to fully fill the resin in thin-wall molding or the like, a high peak pressure sometimes may be needed in the injection and packing processes.
If the peak pressure is excessive, however, breakage of the heating cylinder and/or the mold, burrs on molded products, and other failures are caused. Therefore, a technique is needed to fully fill the resin into the mold without generating an excessive peak pressure.
Techniques to fully fill the resin with stability are disclosed in the following prior art documents.
Japanese Patent Application Laid-Open No. 2-178014 discloses a technique in which a filling process provided between a velocity process and a packing process is started when it is detected that the filling velocity is reduced to 50 to 60% of a set velocity in the velocity process and that the load pressure is increasing, as shown in FIG. 5. In this technique, the filling process is switched to the packing process when the screw velocity is reduced to a low level and the increased load pressure reaches a set filling pressure.
Japanese Patent Application Laid-Open No. 2-274522 discloses a technique in which pressure control is performed with a packing pressure (P2) higher than a switching pressure (P1) for a predetermined period of time after the passage of a preset position by an injected molten material extruding member and the attainment of a preset switching pressure by the injection pressure, as shown in FIG. 6.
In the prior art techniques described above, the necessary peak pressure for filling a resin into a cavity of a mold is reduced. The filling of the resin into the cavity of the mold is stabilized by generating an injection pressure higher than that for the injection process after the end of the injection process (velocity process). If an injection pressure higher than that for the injection process is generated, however, the possibility of breakage of a heating cylinder and/or the mold, burrs on molded products, and other failures increases.